The present invention relates to a method and apparatus for nondestructively measuring the radioactivity of the content in a radioactive waste storage container (referred to merely as "storage container" hereinafter) such as a drum, and more particularly, it relates to a radioactivity measuring method and apparatus adapted to easily measure the radioactivity of .gamma.-ray radiation nuclear material having unknown density and radioactivity distribution stored in a storage container.
Methods of measuring the radioactivity the contents of a storage container having an unknown density have already been proposed, as disclosed in the Japanese Patent Laid-Open Nos. 56-115,974 and 61-107,183. In the conventional radioactivity measuring method disclosed in the Japanese Patent Laid-Open No. 56-115,974, the weight of the contents in a storage container whose radioactivity is to be known is measured, then an average density is calculated by dividing the thus measured value of such weight by the volume of the container, and the radioactivity in the storage container is measured with the use of the calculated average density. However, there has been a problem that this conventional method cannot be used in the case where the density of the contents changes widely in an axial direction of the storage container.
On the other hand, in another conventional radioactivity measuring method as disclosed in the Japanese Patent Laid-Open No. 61-107,183 the density of the contents in the storage container is obtained with the use of a radiation detector and an external .gamma.-ray source which face each other with a storage container therebetween and the radioactivity of the contents in the storage container is measured with the use of the thus obtained density. In this case, the measurement is performed in each of cross-sections (of the container) having different axial heights determined by a collimator positioned in front of the radiation detector, and the whole radioactivity in the storage container is determined by the result of such measurements. Accordingly, in this conventional method, in order to discriminate the .gamma.-ray radiated by the contents in the container from the .gamma.-ray radiated by the external .gamma.-ray radiating source, for each of the predetermined cross-sections of the container, two radiation measurements with and without the external .gamma.-ray source must be performed, thus causing a problem that it takes a relatively long time to measure the whole radioactivity in the storage container.